1. Field of the Invention
The present invention relates to a method of flank milling, and more particularly, the manufacturing of rotor components of turbomachinery, such as impellers and blisks.
2. Description of the Prior Art
The manufacture of radial turbomachinery, including centrifugal compressor impellers and axial compressor or turbine rotors, was either done by casting or by machining. In the case of a centrifugal impeller, casting is the most common method. However, there are well-known problems associated with casting, such as shrinkage and distortion of thin blade sections. The resulting inaccuracies in casting of the blades on an impeller make it impractical to consider fine tuning of the designs since manufacturing errors, in fact, exceed any such changes. As far as milling or machining an impeller blade from a solid block such as a titanium block is concerned, it has been known to use a point milling system on a multi-axis milling machine whereby the surface of the blade is predetermined and each minute area of the surface is machined by the tip of a drill bit. Such milling machines are numerically controlled, and the programs or tapes for operating point milling of an impeller, for instance, is, as can be readily understood, intolerably long and the machining process is time consuming.
Attempts have been made in the past to use flank milling techniques. It is generally conceived that a surface is flank millable if it can be closely approximated to a surface generated by a straight line or a ruled surface. Even given such a surface, the problems of defining the tool path and the cutter feed rate are complex. To complicate the problem further, the milled surface may deviate from the ruled surface, sometimes quite significantly, owing to the twist of the surface along a straight line component. Such deviations have hitherto been ignored or minimized by compromising the aerodynamic design of the blade.
In spite of such difficulties, flank milling has been increasingly used since it offers improved productivity relative to point milling. As described in "A Software System for the Automated Numerical Control Machining of Radial Turbomachinery", a brochure published by Northern Research and Engineering Corporation, of Woburn, Mass., flank milling can lend itself to the manufacture of centrifugal impellers for aviation turbomachinery since the blades' surfaces of such impellers can be designed by straight line generation or ruled surfaces without significant compromise of the aerodynamic design. On the other hand, axial compressor rotors hardly lend themselves to flank milling because of the twist in the blades. Even though flank milling is now well accepted for the manufacture of impellers, compressor or turbine rotor disks are still manufactured as separate blades and disks (rotor hub). The individually forged blades are attached to the disk with a conventional fir tree root arrangement and are riveted to the disk. Attempts have been made to mill a rotor with integral blades from a solid forged blank giving rise to the coined term "blisk" from the words "blade" and "disk". For the purposes of the present specification, the word "blisk" will be utilized.